1. Field of Invention
This invention pertains to tubular connections and particularly to high quality or premium connections that are subject to high temperatures or temperature cycling and that must retain their pressure sealing integrity even after a plurality of connection makeups.
2. Description of the Prior Art
Tubular products, such as tubings and casings, employed in deep hole applications in the oil and gas industry, are subjected to extremely harsh environmental conditions. The connections or joints employed in a tubular string must be suitable to withstand the conditions to which such tubular products are subjected. Among these severe conditions are the extremely high temperatures and pressures that exist and the fact that the connections often have to be repeatedly made up (assembled) and broken out (disassembled).
A connection of the type described in the prior art typically includes one or more thread sets. If more than one set is included there is included a step therebetween. The connection also includes one or more metal-to-metal conical sealing surfaces between the box member of the connection and the pin member of the connection that are subject to large relative motion while in contact during the time that the connection is being made up and broken out. That is particularly true for the sealing surfaces that have a shallow taper or a low angle profile. In such case, the contact duration is long and the bearing stresses are high. Both of these factors tend to subject the surfaces to galling.
Typically, the conical sealing surfaces just described are provided on either side of the thread set or sets and, for a stepped thread configuration, in the area between the thread sets. Sealing does not necessarily occur at all of these locations, but does occur at at least one of them. The surfaces where a seal is effected have to be generally smooth, normally less than 63 R.sub.a (when burnished) in circular texture or finish, since a rougher texture than this will tend to leak gases under the very high pressures that exist. To further prevent leakage and to lubricate the connection, a suitable thread compound composed of lubricants and solid particles to act as dams to "plug" leakage paths in such a connection is generally employed in accordance with API standards as set forth in API BUL 5A2 published by the American Petroleum Institute, especially with metal-to-metal seals that are not required to be highly reliable with only a metal-to-metal sealing action.
Although such smooth sealing surfaces treated with thread compound are suitable for making a single connection, when the connection is broken out and made up again, the sealing surfaces tend to become more slick as the surfaces polish (in a sense, burnish) and the thread compound is baked and squeezed out and scraped off. This action exposes the surfaces to galling. Once a sealing surface is galled, then it is subject to leakage under pressure.
It has been discovered that galling can effectively be reduced or eliminated by texturing one or more of the mating surfaces or areas where sealing occurs with minute scallops that ensure the retention of thread compound even after a threaded connection is made up and broken out many times. Other surface treatment such as with phosphate coatings are also employed to reduce galling tendencies of connections and frequently multiple anti-galling devices are used simultaneously.
The texturing that has been done in the prior art for the purpose of reducing the occurrence of galling is shown, for example, in area 12 of the structure shown and described in U.S. Pat. No. 3,224,799, Blose, et al., issued Dec. 21, 1965. In the Blose, et al. structure, the taper of the conical sealing surface of the pin member is assumed to be the same as the mating conical sealing surface of the box member. The texturing of the surfaces are helical in nature and at a very short pitch compared with the thread pitch. The helical texturing is sometimes referred to as "phonograph finish" texturing or merely phonograph texturing. In cross-section, such a surface will appear as minute scallops. Thus, during each make up of the connection, the so-called threads of the scalloped surfaces are in fact cross-threaded and form a series of minute pockets that trap thread compound. This trapped compound prevents galling each time the connection is broken out and made up again.
Although the "phonographic finish" is successful in reducing galling of conical seals, it has been demonstrated that the undammed valleys between the peaks are sources of leakage paths for high pressure gases, particularly when the connections are subject to thermal cycling. Thus, "phonographic finish" metal-to-metal seals when used with API thread compounds do not display the high reliability of pressure integrity demonstrated by higher grade conical seals with smoother surface finishes. However, the helical texturing of the phonographic finish does entrap lubricants which prevents or reduces the galling of metal-to-metal conical seals as they slide against each other during make up or break out.
Conical seals are preloaded in the radial direction as the pin is threaded axially into the box during makeup. The preload on the conical surfaces provides the barrier to leakage of fluids or gases across the seal. As the conical surfaces are threaded together, deflection of the sections occurs which causes the high preloads to be distributed at various locations of the conical seal and contributes to more severe galling conditions to occur on the pin surfaces. When the pin and box conical seals are fabricated with the same conical angle and assembled, the deflection of the less rigid pin causes the point of rubbing contact to move axially along the conical seal so that no point on the seal is subjected to galling loads for a long helical arc of contact as shown by the illustrations of FIGS. 2 and 3. In FIG. 2, the pin member and the box member are shown just prior to contact. Points A and B on the pin member are at the extreme ends of the area of contact, point B is at the nose end of the area and point A is at the opposite end. Points A' and B' are on the box member surface and are the points that come into initial contact respectively with points A and B. As the connection is tightened, as shown in FIG. 3, the pin member conical surface is deflected (the amount of deflection being greatly exaggerated for illustration purposes) so that a zone AC is in rubbing contact with the box member, point C being intermediate AB. Point B is not in contact with the box member. Thus, the contact area is reduced and all radial preloads cause the bearing stresses to rise, which results in a burnishing action to make rough surfaces smooth, and thus they effectively and reliably seal high pressure gases.
When the pin and box are fabricated with dissimilar conical angles and assembled, the leading edge of the seal is in contact for a longer helical distance during make up and is subjected to more severe galling conditions as shown by the illustration of FIGS. 4, 5 and 6. In FIG. 4, the pin member and the box member are shown just prior to contact. Again, point B on the pin member is at the nose end of the conical area and point A is at the extreme opposite end. Points A' and B' are the initial contact points on the box member corresponding respectively to points A and B. Because of the dissimilar angles of the pin and box members, during assembly as shown in FIG. 5, point B continues to rub as the pin deflects so that at an intermediate position in the assembly process, zone BC of the pin surface is in contact, point C being intermediate AB. Point C' is the point on the box member corresponding with point C. At final assembly, shown in FIG. 6, point A has made contact and rubbed a short distance as the final seal surfaces become substantially parallel. The entire zone AB of the pin member is in contact with the box member. The resulting metal-to-metal seal contact of originally dissimilar angles is a longer zone of contact than for the connection shown in FIGS. 2 and 3, which, even if minor discontinuities in the surface occur and offer a texturous labyrinth to gases attempting to leak across the seal, the contact zone is nevertheless leak tight for practical purposes.
Therefore, it is a feature of the present invention to provide an improved thread connection that effectively seals under such high temperature or temperature cycling conditions and in the presence of such high pressures and does not gall when subject to multiple make ups and break outs.
It is another feature of the present invention to provide an improved thread connection that has the seal area of the pin at a slightly less taper than the seal area of the box and that includes phonograph finish texturing ahead of the smoother point of seal makeup contact on the pin member of the connection so as to provide retention of thread compound in the scallops of the texture and so as to permit metal-to-metal sealing behind the textured surface.
It is still another feature of the present invention to provide in one embodiment of an improved thread connection, a slightly convex surface, which is also textured, just ahead of the point of seal makeup contact on the pin member of the connection so as to provide enhanced lubrication during stabbing and make up.
It is yet another feature of the present invention to provide another embodiment of an improved thread connection that has landing contact areas of pin and box at the same taper in front of the seal area on the pin, the seal area on the pin being at a lesser angle than the box seal area, the landing area on the pin being suitably textured for a short distance for thread compound retention and immediately followed by a smoother surface conical seal of higher grade pressure integrity for improved sealing reliability.